$directions:("t":"top", "b":"bottom", "l":"left", "r":"right");
$dimensions:("p":"padding", "m":"margin");
//获取padding margin间隔
@each $dim in $dimensions {
    //循环四个方向
    @each $dir in $directions {
        @for $i from 1 through 20 {
            $size: $i*5;
            .#{nth($dim,1)}-#{nth($dir,1)}-#{$size} {
                #{nth($dim,2)}-#{nth($dir,2)}: #{$size}px;
            }
        }
    }
}